Frank Sinatra and Dean Martin got into a barroom brawl in Beverly Hills that nearly left someone dead – and the details remain murky decades later.

Author and longtime People magazine contributor Scott Huver has written a new book, "Beverly Hills Noir: Crime, Sin, & Scandal in 90210," that explores some of the most outrageous incidents to occur in the star-studded city.

"These two guys were at the very top of the entertainment food chain," Huver told Fox News Digital. "They couldn’t have been more popular; they couldn’t have been more well paid and more iconic in 1966 when the incident happened."

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Dean Martin holding a cigarette and sitting next to Frank Sinatra as they both look concerned.

A 1966 barroom brawl nearly derailed the careers of Dean Martin (left) and Frank Sinatra (right). (Martin Mills/Getty Images)

"For something like this to throw a monkey wrench into their lives… something as simple as a couple of cross words and racial epithets thrown around in a bar [led] to fisticuffs that ultimately threatened their careers should things take a really dark turn," said Huver. 

"It’s a very little explored facet of both men’s careers. It hinges on one night in both of their lives that should have been a celebration for Dean’s 49th birthday."

On the night of June 8, 1966, Sinatra and Martin were at the Polo Lounge alongside Sinatra’s bodyguard/pal, Jilly Rizzo, actor Richard Conte, as well as plenty of ladies.

Sitting next to them at a booth was Frederick Rand Weisman, a prominent art collector and the former president of Hunt’s Food, who was accompanied by businessman Franklin H. Fox. The men were barely sitting for 10 minutes when Weisman grew increasingly annoyed by the "rowdy laughter" of the celebrity guests.

Book cover for Beverly Hills Noir

Scott Huver's book, "Beverly Hills Noir: Crime, Sin, & Scandal in 90210," is available now. (Post Hill Press)

That’s when Weisman got up and did something few people dared to do: ask Sinatra and his pals to quiet down.

Sinatra claimed that Weisman told him, "You talk too f------g loud, and you have a bunch of loudmouthed friends," before using "a certain d-word and a certain w-word that Italian Americans typically object to," Huver wrote. 

Some claimed that Weisman also scolded Sinatra for using foul language in front of ladies.

Sinatra allegedly responded with an antisemitic remark. He later claimed Weisman decked him in his right eye before slipping and falling, breaking the base of a cocktail table as he crashed to the floor. 

Fox claimed to Sinatra biographer Kitty Kelly that the singer stormed out of the room when things came to blows.

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Frank Sinatra punching a man.

Frank Sinatra's fiery temper was well-known in Hollywood. (Getty Images)

According to reports, Fox and a hotel security guard tried to keep the two men apart as Martin pleaded, "Let’s get out of here, Frank!" That’s when Sinatra grabbed one of the boothside telephones and hurled it at Weisman, knocking him out cold.

According to Huver’s book, it’s also possible that after everyone lunged at each other, Weisman slipped and fell. What’s known for certain is that Weisman "had landed on the floor and was flat on his back amid an upturned ashtray, a cast-off tablecloth, and a clutter of broken crystal. It sounded as if he were snoring. And he wasn’t getting up."

"What we know for sure was that… [Weisman] objected to some of the language that was being used by Frank and Dean, and it turned into a typical barroom confrontation," Huver explained to Fox News Digital.

"What we don’t know exactly is who swung first, who did what to whom in what order. But it ended up with somebody clobbering… Frederick Weisman over the head with one of the famous pink telephones that sat boothside on all the banquettes in the Polo Lounge. He ended up unconscious on the floor."

Frank Sinatra playfully punching Dean Martin

It's still questioned today who gave the near-fatal blow. (Martin Mills/Getty Images)

"Now, he revived, but within the day, he was comatose again," Huver continued. "He was in the hospital and then underwent surgery. And it could have been particularly bad. He might’ve died. That’s when Frank and Dean… separately left Los Angeles to… wait out the news while the police investigated. That’s where we get all of these conflicting stories where nobody exactly saw what happened."

"Nobody gave the same story," Huver added.

According to the book, Weisman endured a nearly three-hour operation to alleviate a skull fracture. Doctors couldn’t guarantee that he would survive.

Huver said Sinatra fled to Los Angeles where he was "sweating it out" with his much younger girlfriend, actress Mia Farrow. Martin went to Lake Tahoe to play golf. He "had nothing to say" when he was questioned by police.

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Frank Sinatra sitting on an orange couch holding a cigarette.

"Was he mobbed up? I think there’s a lot of debate to be put on that," said Scott Huver about Frank Sinatra's alleged Mafia ties. (Getty Images)

"Frank was 50 at the time," said Huver. "He was grappling a little bit with how huge he was, how iconic he had become, how rumors dogged him. And then he was involved with somebody… half his age. So there was a lot of stress on him even though he was on top of the world. And I think that occasionally expressed itself in outbursts. He was an emotional guy."

"Was he mobbed up? I think there’s a lot of debate to be put on that," Huver shared. "Was he a violent character? I don’t know if that’s true either. But there are certainly incidents that suggest that if he lost his temper, things could go poorly."

Over the years, rumors also persisted that it was Rizzo, not Sinatra, who gave Weisman the near-fatal blow by hitting him over the head with a telephone.

Frank Sinatra walking in between Jackie O and Jilly Rizzo.

Jacqueline Kennedy Onassis, Frank Sinatra and bodyguard Jilly Rizzo arrive at the 21 Club after Sinatra's concert circa 1975 in New York City. (PL Gould/IMAGES/Getty Images)

"Jilly Rizzo… was Frank Sinatra’s best and most loyal friend, more so than any of the more famous figures in the Rat Pack," said Huver. "Jilly was at his side pretty much until the end of Jilly’s days. They had a very, very special bond. They were like brothers. And Jilly came from the New York nightclub world.

Sammy Davis Jr. laughing with members of the Rat Pack wearing matching tuxes

Jan Murray, left, sits alongside Rat Pack members Dean Martin, Sammy Davis Jr. and Frank Sinatra as the group unwinds backstage at Carnegie Hall after entertaining at a benefit performance in honor of Dr. Martin Luther King Jr. (Getty Images)

"He had his own restaurant. He was built like a tractor-trailer. He kind of appointed himself as Frank’s... bodyguard. And so, if trouble started, it is very easy to imagine that Jilly was the one to step in and handle it… There were subsequent incidences in years ahead in which Jilly did the exact same thing."

"Was it because he was a violent man? Everybody in Hollywood who knew Jilly seemed to absolutely adore him and be charmed by him," Huver shared. "But he was not afraid to take a stance if he thought his best friend Frank Sinatra was in trouble or being threatened."

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Rizzo repeatedly denied ever being Sinatra’s bodyguard in any official capacity.

"Sinatra don’t need no protection," said Rizzo, as quoted in Huver’s book. "He’s man enough to stand up and defend himself in his own way, like any man should."

Sinatra insisted to detectives that he didn’t initiate the fight.

Frank Sinatra surrounded by photographers.

It wouldn't be the last time Frank Sinatra found himself in hot water before his death in 1998. (Getty Images)

"The guy was cursing me and using four-letter words," he told investigators, as quoted in Huver’s book. "I told him ‘I don’t think you ought to be sitting there with your glasses on making that kind of conversation.’ The guy got up and lunged at me. I defended myself, naturally."

Sinatra contended that after Weisman slugged him, Fox tried to separate them. That’s when Weisman fell without anyone laying a finger on him.

"I at no time saw anyone hit him — and I certainly did not," said Sinatra. 

He noted that on his way out he "saw a man on the floor."

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Frank Sinatra and Dean Martin looking concerned as they stand next to each other.

Dean Martin and Frank Sinatra attempted to keep a low profile as Frederick Rand Weisman recovered. (Mondadori via Getty Images)

Weisman eventually gained consciousness but was still suffering from confusion and memory loss. Police, who listed Sinatra as their prime suspect, were eager to interview Weisman.

Weisman told them that he could remember everything leading up to the altercation "but nothing else." The whole incident was a blank to him. Doctors called his condition "retrograde amnesia."

According to the book, Weisman’s family "had been incensed" and wanted to press assault charges against Sinatra. However, they "thought better of it." Kelly claimed a family member told her they were receiving anonymous telephone threats "from ominous-sounding figures."

Hollywood attorney Grant Cooper announced that the Weismans wanted to forget the incident and move on. The detectives, unable to do more, closed the case.

Frank Sinatra and Dean Martin getting kissed by two women.

Scott Huver tracked down Frank Sinatra's publicist to find out what really happened that night. (ullstein bild via Getty Images)

For the book, Huver was able to track down Sinatra’s publicist, a surviving player in the saga.

"According to Frank’s publicist, money was exchanged," said Huver. "He was supposed to be there with the group celebrating that night and made it a habit not to be involved. So, if anything did go badly, he was in a good position to clean things up. He said that the patrons, the bartenders, waiters – all the staff at the Polo Lounge all received some kind of compensation to keep their stories very vague."

Weisman was plagued by amnesia for months. Still, he did go on to live a long, prosperous life, dying in 1994 at age 82.

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Frank Sinatra putting his arm on Mia Farrows shoulder.

Frank Sinatra and Mia Farrow were married from 1966 to 1968. (Getty Images)

An elated Sinatra, who realized that the whole ordeal wouldn’t derail his career, swiftly purchased an $85,000 nine-carat engagement ring. Sinatra and Farrow married in 1966. 

According to Huver’s book, "Dean Martin got the thankless task of informing the Sinatra children — so much closer in age to Mia than their father was — of the nuptials just as the ceremony was taking place."

The couple divorced in 1968.

What happened at the Polo Lounge remains shrouded in mystery. Still, Huver has his theory.

Frank Sinatra and Dean Martin looking concerned at a party.

Frank Sinatra (right) and Dean Martin (left) remained friends until Martin's death on Christmas Day 1995. (Jack Albin/Hulton Archive/Getty Images)

"There are… rumors that Frederick Weisman… and his family may have received a payout because they never chose to press charges," said Huver. "That’s speculation, but it does sound like it was papered over with green paper."